Doctors and other health care professionals nationwide have been victimized this year by identity theft used to fraudulently obtain federal and state tax refunds — and their numbers are growing, medical industry and government officials say.

The American Medical Association says the exact number of doctors affected by tax fraud isn’t known, but hundreds of cases have been confirmed in Vermont, New Hampshire, Connecticut, Indiana, Maine and elsewhere.

The number of doctors, dentists and nurses targeted by identity theft scams has increased significantly this year, raising questions about a possible security breach of the personal information of medical workers, U.S. Sen. Richard Blumenthal, of Connecticut, said Thursday.

“It is incredible. The scope and size are appalling and astonishing. ... We’re talking about hundreds (of doctors) nationally and tens of millions and maybe hundreds of millions of dollars,” Blumenthal said.

Blumenthal and fellow Connecticut Democrat Sen. Christopher Murphy urged the IRS and Secret Service to dedicate their resources to solve these crimes and prosecute the perpetrators. Their pleas followed the rejection of many doctors’ federal and state tax returns last month because they had been filed by someone else. At the time, U.S. Sen. Jeanne Shaheen, a Democrat from New Hampshire, also called for an investigation.

More than 150 doctors and health care providers in New Hampshire and Vermont and at least 35 Maine physicians reported being victims of tax fraud. More than 300 dentists in Michigan say they were tax fraud victims, the Detroit radio station WWJ-AM reported this week.

Both the IRS and the Secret Service said Thursday they are looking into identity fraud involving the medical profession, but IRS officials note that many Americans, not just doctors, fall prey to tax fraud.

“Identity theft is one of the fastest-growing crimes nationwide, and refund fraud caused by identity theft is one of the biggest challenges facing the IRS,” the agency said in a statement, adding that it has stopped 15 million suspicious tax returns and more than $50 billion in fraudulent refunds since 2011.

Attorney General Eric Holder also said recently that the “scale, scope and execution of these fraud schemes” has grown substantially. He said that in the past year, the Justice Department has brought charges against 880 people.